ATM is a switching and multiplexing technique designed for transmitting digital information, such as data, video, and voice, at high speed, with low delay, over a telecommunications network. The ATM network includes a number of switching nodes coupled through communication links. In the ATM network, bandwidth capacity is allocated to fixed-sized units named “cells.” The communication links transport the cells from a switching node to another. These communication links can support many virtual connections, also named channels, between the switching nodes. The virtual connections, for example a Virtual Channel Connection (VCC) or a Permanent Virtual Circuit (PVC), assure the flow and delivery of information contained in the cells.
The ATM Forum, which is a user and vendor group establishing ATM standards, has also defined several ATM service categories, used in characterization of a virtual connection. For example, among such service categories are (1) a Constant Bit Rate (CBR), which supports a constant or guaranteed rate to transport services, such as video or voice, as well as circuit emulation, which requires rigorous timing control and performance parameters; (2) a Variable Bit Rate (VBR), real time and non real time, which supports variable bit rate data traffic with average and peak traffic parameters; (3) an Available Bit Rate (ABR), which supports feedback to control the source rate in response to changed characteristics in the network; and (4) an Unspecified Bit Rate (UBR).
FIG. 1a illustrates a prior art packet network 100, typically including several network nodes, also known as switching nodes, 110 connected through single communication links 120. The packet network 100 is a data transmission network with guaranteed bandwidth and quality of service. Typically, end users 130 access the network 100 and connect to the nodes 110 via similar links 120. Generally, the illustrated communication links 120 carry traffic from many sources to many destinations and may support multiple virtual connections. Although these virtual connections may be statistically multiplexed onto the same link, the network 100 must still meet certain quality of service requirements for each connection.
A failure within the network 100 will interrupt the flow of data from a source end user to a destination end user. When the flow of data is interrupted for a longer period of time, typically for a period longer than 250 milliseconds, some voice communications, such as voice calls, carried by the network will be dropped. In order to improve the reliability of networks, several systems have been designed to eliminate such extended data interruptions.
One such system is an automatic protection switching (APS) system. As illustrated in FIG. 1b, in the packet network 100, parallel links 122, 124 connect the nodes 110 and are used to transmit duplicate information between the nodes 110 and to ensure fast and reliable data transmission. Link 122 is called an “active” link, while link 124 is a “stand-by” link. Because the same information is transmitted on both links 122, 124, the switching node 110 located at the receiving end can choose either link to receive the transmitted information. For example, if the active link 122 fails, the stand-by link 124 can deliver the same information to the switching node. This APS configuration can be implemented, for example, with the SONET/SDH standards, and can also be used to transport data packets instead of voice communications.
Although the APS system addresses link failures and can switch to a redundant link within 250 milliseconds, it cannot solve a network failure across multiple nodes, unless the network is covered with the APS system, which could be very expensive. Also, although the APS system increases the reliability of packet networks, the duplicated data sent on both links 122, 124 reduces the bandwidth in half, resulting in a waste of bandwidth.